Research on the application of sagnac interference combined with multimode interference in vibration measurement

Abstract

In order to further improve the frequency discrimination ability of fiber optic vibration sensor, this paper uses the multimode fiber (MMF) coupler to construct the Sagnac interferometer, which combines the multimode interference with Sagnac interferometer. Multimode fiber couplers with splitting ratio of 1:1 were fabricated by using MMFs with core diameters of 105 and 62.5 μm respectively, and the Sagnac loops were constructed with the couplers. For comparison and analysis, single-mode-multimode-single-mode interference structures and Mach-Zehnder interferometer were also fabricated by MMFs respectively. Furthermore, single mode fiber based Sagnac loop was also fabricated. Through a large number of experiments, these structures were used to measure vibration frequency, and their performance were compared in detail in the frequency range and the frequency accuracy of vibration detection. The results showed that the intensity difference between the primary peak and the secondary peak of the frequency spectrum obtained by the Sagnac interferometer with the core diameter of 105 μm step-index MMF is greater than 20 dB in the range of 0.3 to 20 kHz with frequency deviation in the order of Hz. With the large detectable frequency range and frequency recognition capability and higher sensitivity to the low frequency vibration, the Sagnac interferometer combined with multi-mode interference provided by the 105 μm step-index MMF is a kind of structure with the best performance among these studied structures in vibration measurement, which provides a new scheme for the research of vibration detection.